The present invention relates to nonaqueous electrolyte secondary batteries having positive-electrode active materials and negative-electrode active materials, which intercalate/deintercalate (or are doped/undoped with) lithium, and nonaqueous electrolyte solutions.
Nickel-cadmium batteries and lead batteries have been used as secondary batteries for electronic devices. Trends toward higher performance and miniaturization of electronic devices due to advanced electronic technology require secondary batteries having higher energy densities. Since nickel-cadmium batteries and lead batteries have low discharge voltages, increases in the energy densities are limited.
Nonaqueous electrolyte secondary batteries using carbonaceous materials capable of intercalating/deintercalating lithium in negative electrodes and lithium compound oxides in positive electrodes have been vigorously developed instead of the nickel-cadmium batteries and lead batteries, since the nonaqueous electrolyte secondary batteries, called lithium ion batteries, have high discharge voltages and reduced self-discharge, and have prolonged cycle lives.
In these nonaqueous electrolyte secondary batteries, carbonaceous materials such as graphite are used as negative-electrode active materials, LixMO2 wherein M is at least one transition metal and 0.05<x<1.10 is used as positive-electrode active materials, and LiPF6 and LiBF4 are used as electrolytes. As organic solvents for dissolving the electrolytes, propylene carbonate, ethylene carbonate, γ-butyrolactone, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, ethyl acetate, methyl propionate, 1,2-dimethoxyethane, and 2-methyltetrahydrofuran are used.
The nonaqueous electrolyte secondary batteries are suitable as power sources for portable electronic devices. In recent years, compact battery packs including batteries and protective circuits have been frequently used with requirements for reduced sizes and weight. In the batteries in the battery packs, higher capacities at large-current discharging modes are required. In order to fulfill such a requirement, improvements in negative electrodes are essential in addition to improvements in positive electrodes and nonaqueous electrolyte solutions.
Current nonaqueous electrolyte secondary batteries, however, are still unsatisfactory as regards improvements in capacities during large-current discharging modes by improvements in negative electrodes.